The combined fury of Hurricane Irene and Tropical Storm Lee may have drownedmuch of the region's stink bug population, but scientists are still hesitant to say that homeowners will see fewer of them when the weather warms.

Scientists say something caused a substantial decline in the number of the bugs last fall before they hunkered down in the region's attics and closets. Perhaps it was due to natural predators or an unknown parasite. Just as likely, they say, it was the deluge that began just before Labor Day and lasted through September.

"We suspect it was heavy rain because of the regional effect on them," said Tracy Leskey, research entomologist for the U.S. Department of Agriculture. "The population is substantially lower, but we don't know the reason definitely."

From Aug. 31 through September, the Middle-Atlantic region received more than 16 inches of wind-driven rain, four times the average for the period.

"That knocked bugs off trees and crops and it probably drowned a large number of them," said Jerry Burst, a pest management and vegetable specialist at the University of Maryland's Central Maryland Research and Education Center in Upper Marlboro. "That contributed to the decline."

Particularly vulnerable were nymphs, early-stage stink bugs, scientists said. Instead of having two generations produced in a single season, as happened in 2010, the second generation of 2011 may have gotten a late start that could prove fatal to the bugs.

"Irene and heavy rain suppressed the population of nymphs. They got cold and wet and it's possible secondary disease had its effect," said Stanton Gill, a specialist with the University of Maryland Cooperative Extension.

Scientists acknowledge they are just making an educated guess. But before winter arrived, something reduced the region's overall population last year, although pockets of heavy infestation were reported in places such as Columbia and Deale.

Brown marmorated stink bugs, natives of Asia, have been found in 33 states, according to the U.S. Department of Agriculture. They attack ripening fruits and vegetables, creating rotted pockets that make the produce unsalable. When crushed, the dime-sized pests emit a skunk-like odor that lingers long after an amateur exterminator has tossed the carcass in the garbage or flushed it away.

Stink bugs don't have a long track record in the region. The first confirmed sighting in Pennsylvania was in 2001 and Leskey was credited with discovering the first stink bug in Maryland in 2003. The public took notice in 2010, when the infestation was intense and extensive. But last year was, by comparison, mild.

"Now, the question is, 'Why?'" said Mike Raupp, a University of Maryland entomology professor. "I think maybe 2010 was the abnormal year and 2011 was normal."

As a result, scientists are unwilling to predict the misery index that farmers and homeowners will experience this year as the weather warms and the bugs become active.

"Any decline is welcomed. But there's still plenty of them out there," said Leskey.

The mild winter could be a harbinger of an extensive infestation like in 2010. But if a cold spell hits the region after the recent warm weather, the emerging bugs could suffer from malnutrition, "flip over and die," Raupp said.

Meanwhile, a task force of 51 researchers at 14 institutions in 10 states is trying to unlock the stink bug mystery. Working at the agriculture department's Appalachian Tree Fruit Research Station in West Virginia, Leskey is hoping to isolate a male-produced pheromone that attracts stink bugs of both genders.

"Bug chemistry can be fairly complicated in the types of compounds they produce. Once this is done — and we'll get there, for sure — it can be used to enhance traps," Leskey said.

Pheromone-baited traps will be used to get a better picture of population size, mating, movement and trends. Later, scientists may use the scent to attract bugs to one spot for eradication, eliminating the need for wide-scale spraying.

"This isn't something that will be ready in 2012. It's down the road and we are working as quickly as we can," Leskey said.

Other scientists are investigating whether tiny parasitoid wasps could help destroy stink bug eggs and what types of plants — such as the Tree of Heaven — act as natural attractants and should be removed.

"We're in the midst of a great experiment," said Raupp. "We're totally in uncharted waters."